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Homework Help: Kinetic Theory

  1. Jun 14, 2010 #1
    1. The problem statement, all variables and given/known data

    From the average separation between air molecules at STP, and their mean speed, estimate how long it would take one molecule to move into the region occupied by another. Assume that air consists mainly of nitrogen molecules.

    2. Relevant equations
    1/2mv^2 = 3/2KT

    3. The attempt at a solution

    Distance between molecules at STP:

    (6.02 x 10^23) x D^3 = 22.4 x 10^-3 m^3
    D^3 = 3sqrt 22.4x10^-3 / 6.02 x 10^23
    D = 3.34 x 10^-9 m

    Since mass of N2 = 28 g/ mol, I used this to find the average velocity of the molecules:

    1/2mv^2 = 3/2KT
    v(rms) = sqrt(3KT/m)
    = sqrt (3K (273) / .028 kg
    = 6.35 x 10^-10 m/s

    I then just plugged my two answers from above into the velocity equation,
    velocity = distance/time
    time = D/V
    time = 3.34 x 10^-9 / 6.25 x 10^-10 = 5.3 seconds

    Does what I did make sense? The velocity looks awfully small, so I'm not sure. Any help appreciated!
  2. jcsd
  3. Jun 14, 2010 #2
    The "m" is the mass of each molecule. You've noted that the mass for each mole is 0.028 kg. You need to divide this by 6.02E23 to get the mass per molecule.
  4. Jun 14, 2010 #3
    Ah ha! Good eye, thank you for that.

    My new velocity is 493 m/s, and my new time is therefore 6.77 x 10^-12 seconds.

    Do these numbers make sense?
  5. Jun 14, 2010 #4
    I think so. At first I was thinking your distance was way too small (it's on the order of the atomic width). But I can't see anything wrong with logic. So I googled "average separation of atoms in a gas" and got the following link:

    http://www.practicalphysics.org/go/Guidance_34.html;jsessionid=alZLdQlAHb1?topic_id=4&guidance_id=1 [Broken]

    and one from here:


    Both confirm your number.
    Last edited by a moderator: May 4, 2017
  6. Jun 14, 2010 #5
    Thanks very much for all your help
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